More and bigger drilling-linked earthquakes rattle Oklahoma

OKLAHOMA CITY, July 27 (Reuters) - Several earthquakes shook
Oklahoma on Monday as the state experiences a sharp increase in
the frequency of tremors linked to wastewater disposal from gas
and oil drilling, including from fracking, state and federal
officials said.

Three of Monday's quakes measured above a magnitude 4.0,
with a 4.5 earthquake centered just north of Crescent, roughly
45 miles (72 km) north of Oklahoma City, the U.S. Geological
Survey (USGS) said.

The largest tremor, logged a "significant earthquake" by the
USGS, could be felt as far away as Wichita, Kansas, about 160
miles north, broadcaster KOTV reported.

There were no reports of damage.

The rate of earthquakes in Oklahoma has increased by about
50 percent since 2013, greatly increasing the chance for a
damaging quake, according to the USGS.

Noticeable quakes - above magnitude 3.0 - now hit the state
at a rate of two per day or more, compared with two or so per
year prior to 2009. During the past seven days, Oklahoma has
experienced about 40 earthquakes, according to the USGS.

Scientists say the seismic activity is triggered by the
injection of wastewater from booming oil and gas drilling
operations into deep geological formations.

The state's oil and gas regulator released a directive this
month expanding "Areas of Interest", parts of the state that
have been worst-hit by the quakes, and adding restrictions for
211 disposal wells.

In March, the regulator - the Oklahoma Corporation
Commission - also directed 347 wells to reduce their injection
depths to above the Arbuckle formation. High-volume injections
into the Arbuckle, the state's deepest formation, have the
highest potential for seismic activity, according to the USGS.

Twenty-one of Oklahoma's 77 counties are under the order,
and oil and gas drilling operators have until Aug. 14 to comply
with reducing injection depth.
(Editing by Victoria Cavaliere and Louise Ireland)